Along with a preference for a three-dimensional structure in a device structure, structures have been studied by sterically stacking substrates (hereinafter, referred to as “wafer”) such as a semiconductor wafer, on which an integrated circuit is mounted, on one another and by electrically connecting the mutually adjacent wafers by wiring metals embedded in the wafers. In this case, a recess such as a hole for embedding the wiring metal in the wafer is formed in each wafer by a plasma etching process. Specifically, the wafer includes a monocrystalline silicon (Si) layer, in which the recess is formed, for example, and a resist mask, which is laminated above the silicon layer and patterned to correspond to the pattern of the recess. Between the silicon layer and the resist mask, an intermediate layer containing, for example, a silicon nitride (SiN) film is interposed to use it as a stopper film in, for example, a chemical mechanical polishing (CMP) process of removing excess metal after embedding the wiring metal in the recess. Therefore, after the silicon layer is exposed on the intermediate layer by etching the intermediate layer, the silicon layer undergoes an etching process.
The depth of the recess may be 100 μm relative to an opening size of, for example, about 10 μm. Therefore, a sidewall of the silicon nitride film is exposed to plasma while the silicon layer is etched by the plasma of the process gas containing, for example, a sulfur fluoride (SF6) gas, an oxygen (O2) gas, and so on. Therefore, the silicon nitride film is side-etched from a side of the silicon nitride film (a side of the recess). Then, an airspace is formed between the resist mask and the silicon film around the recess. When the airspace is formed, the wiring metal intrudes into the airspace in a subsequent process. Then, desired device characteristic features are not obtainable such that a leak current increases.
Patent document 1 discloses a technique of restricting undercut from occurring by forming a protection film on a sidewall of a photo resist film using, for example, plasma of a C4F8 gas.